Traditional antennas used in receiving transmitted signals such as, for example, GLONASS and/or GPS signals, are subject to various error factors which compromise the accuracy and reliability of their resultant position data. One such error factor is inadequate visibility of satellites in some locations, such as in urban canyons where signals from satellites may be obscured by buildings and other obstacles. A second such factor is the problem of reflected and refracted signals resulting in what is known as multipath, the condition of an antenna receiving both direct and reflected signals from one or more satellites.
GPS signals, for example, are circularly polarized in a right-hand path (Right Hand Circularly Polarized RHCP). If the signal path to the antenna includes reflection, such as from the side of a building, for example, this polarization may be inverted to left-hand polarization (Left Hand Circularly Polarized or LHCP) in the reflected portion of the signal. Reflection of a signal may also affect the phase and amplitude of the reflected signal. The reflected component of the combined signal has a longer path to the antenna than the direct signal, and a longer signal travel time. The reflection of the multipath component will weaken the reflected signal depending on the additional travel and the electromagnetic properties of the reflecting surfaces. The signal may also be diffracted by building edges, for example.
When a combination of direct and reflected signals is received by a GPS antenna the combination may be constructive, causing a timing error, or destructive, also causing a timing error. The multipath-induced timing error is proportional to the strength and timing of the multipath signal relative to the direct signal. Despite various design solutions in the construction of antennas to attenuate the multipath component of combined signals, the ability to reduce multipath components to harmless levels has not been achieved. A second aspect of the problem is that multipath parameter estimation is made more difficult by the presence of noise, and this factor may be exacerbated when the multipath signal is partially attenuated.
Accordingly, there is a need in the art to address these and other problems in reception of satellite signals as well as signals from other transmitters.